Conventionally, in order to obtain a lens system suitable for use of a solid-state imaging element, it is known to use an objective optical system which has a long back focus and a small size and which provides improvements in optical performance, such as an improvement in color reproducibility and a satisfactory correction of the image field curvature (see, for example, PTL 1).
The objective optical system disclosed in PTL 1 is used for forming an object image on a solid-state imaging element arranged on a linear optical axis. Therefore, a prism, and the like, which bends the optical axis, does not need to be arranged between the objective optical system and the solid-state imaging element, and hence a certain length of back focus is sufficient for the formation of the object image.